1. Field of the Art
This invention relates to a process for preparing a water absorptive composite comprising a water absorptive polymer and a prefabricated fibrous substrate. More particularly, this invention relates to a process for preparing a water absorptive composite in which a highly water absorptive polymer is held on a prefabricated substrate, comprising applying an aqueous solution containing an acrylic acid type monomer, vinylpyridine type monomer, crosslinking agent and oxidative radical polymerization initiator to a prefabricated fibrous substrate, and then carrying out polymerization at room temperature within a short time with addition of a reducing agent.
The water absorptive composite obtained by the process according to this invention can be advantageously used in the production of a variety of water absorptive materials, because it is excellent in water absorption properties, has a high water absorption rate, and has an extremely low content of unpolymerized monomers, and the highly water absorptive polymer is held with good stability on the fibrous substrate.
2. Prior Art
Materials such as paper, pulp, nonwoven fabric, spongy urethane resins and the like have hitherto been used as water retentive materials for a variety of sanitary goods such as a sanitary napkin, paper diaper and the like and a variety of agricultural materials. However, these materials have a water absorption capacity of no more than 10-50 times their own weight, which will cause problems that an extensively increased bulk of the material is required for absorbing or retaining a large amount of water and that water is easily released from the material in which water has been absorbed on pressing it.
There have recently been proposed a variety of highly water absorptive polymer materials in order to settle the aforementioned problems of the water absorptive materials of this kind. For instance, there have been proposed a graft polymer of starch (Japanese Patent Publication No. 46199/78, etc.), a denaturated cellulose (Unexamined Published Japanese Patent Application No. 80376/75, etc.), a crosslinked water soluble polymer (Japanese Patent Publication No. 23462/68, etc.), a self-crosslinking polymer of an alkali metal salt of acrylic acid (Japanese Patent Publication No. 3070/79, etc.), and the like.
However, these highly water absorptive polymer materials, while having a relatively high level of water absorption properties, are obtained as powder in most cases. Therefore, in order to use them for sanitary goods such as a sanitary napkin, paper diaper or the like, it is necessary to disperse them homogeneously on such substrates as tissue paper, nonwoven fabric, cotton or the like. However, the polymer powder having been dispersed in such a manner is difficult to be firmly held on the substrate and often agglomerate partially. Also, swollen gel after water absorption will easily move from the substrate without being held firmly on it. Therefore, if it is used for a paper diaper, for example, it will give the feeling of stiffness upon urination accompanied with the extremely uncomfortable feeling on wearing. Furthermore, in a process for obtaining an absorber by dispersing such a powdery polymer as described above on a substrate, the absorber will be very expensive because of complicated procedures for powder handling and of problems on processes for efficiently conducting uniform dispersion.
As a method for dissolving these problems, there is disclosed a process for producing a water absorptive composite in which an aqueous solution of an acrylic acid type monomer is applied in a previously determined pattern to a prefabricated fibrous substrate to obtain a composite, which is then irradiated with electromagnetic radiation or corpuscular ionizing radiation to convert the acrylic acid type monomer into a highly water absorptive polymer (Unexamined Japanese PCT Patent Publication No. 500546/82). According to this process, uniform dispersion and stable holding of the aforementioned powder on a substrate are considerably improved. However, since electromagnetic radiation or corpuscular ionizing radiation is employed for converting the monomer into the high water absorptive polymer in this process, the highly water absorptive polymer inherent to the specific monomer tends to be crosslinked excessively. As the result, the composite obtained will exhibit extremely poor properties as an absorber. Especially its water absorption capacity will be of a level of only half or less of that of the composite obtained by using the aforementioned highly water absorptive powdery polymer. Moreover, this process involves problems in respect of safety on operation of a device for the above radiation and also production cost.
More recently, Unexamined Published Japanese Patent Application No. 149609/85 discloses a process for preparing a water absorptive composite material comprising previously impregnating a water absorptive organic material with an aqueous solution of an acrylic acid type monomer and adding thereto in a mist form a water soluble radical polymerization initiator, or, a water soluble radical polymerization initiator and a water soluble reducing agent to conduct polymerization. In this process, however, the water soluble polymerization initiator is added after the water absorptive organic material has been impregnated with the acrylic acid type monomer. Thus, although the polymerization initiator is added in a mist form, it is very difficult to completely polymerize the monomer because of occurrence of "uneven polymerization" and as the result the amount of the residual monomers is in a high level, which will cause problems on safety and lead to lowering of the properties of the resulting product, especially in respect of its water absorption capacity.
Under these backgrounds, the present inventors have already proposed in Japanese Patent Application No. 238421/85 a method that an aqueous solution of an acrylic acid type monomer containing a small amount of a crosslinking agent and an oxidative radical polymerization initiator are previously mixed and the mixture is applied to fibrous substrate, and then an amine or a reducing agent is added to conduct polymerization. It has been found that according to this method there hardly occurs "uneven polymerization", polymerization proceeds very easily and a composite having a large water absorption capacity can be obtained. A further research on this method has revealed that water absorption capacity can be remarkably improved by using a specific polymerizable monomer with the acrylic type monomer and adopting a controlled polymerization condition.